Lasers for use in the Band I range of wavelengths are suitable for many applications such as remote sensing, laser radar, directed infrared countermeasures, and others. Typically, lasers which are capable of working in the Band I range are bulk lasers made from crystals. It would be advantageous to have Band I laser capability using fiber lasers instead of bulk lasers, as fiber lasers are known to be easier to manufacture, more efficient, more stable, more robust, and have a stable and well defined output beam. However, such fiber lasers have been thought to be highly inefficient or impossible to lase at the required wavelengths due to low gain, reabsorption by the active ion, or absorption of the host material.
Furthermore, lasers for use in the Band IV range of wavelengths are useful for many applications as well. For example, Direct IR Countermeasures (DIRCM) are systems that activate a directional jamming means against an incoming missile. An example of a DIRCM system is described in greater detail in WO 2004/109323, incorporated by reference herein in its entirety.
An ideal source for producing Band IV energy for DIRCM or other applications would be a fiber laser having a large pulse energy. However, fiber lasers with high powers (kW CW) and moderate (mJ) pulse energies (such as, for example, double clad fiber lasers) at Band IV wavelengths do not exist or are not practical for engineering. Specifically, Band IV is generated by converting an available wavelength. For example, an optical parametric oscillator (OPO) based on periodically poled lithium niobate (PPLN) can convert the output wavelength of a Yb fiber laser from 1 μm to 4 μm with an efficiency of about 10%. In order to obtain a 4-5 W output, a 40-50 W laser must be used. However, the characteristics of the fiber (such as a 20/400 Yb fiber) dictate a fiber length of approximately 8 m, which can result in nonlinear effects such as stimulated Brillouin scattering (SBS), stimulated Raman scattering (SRS), and self-phase modulation (SPM) if high energy (mJ level) pulses are generated. Furthermore, the OPO will not be able to handle generation of 4 W at 4 μm, particularly since the PPLN absorbs this wavelength, resulting in high thermal effects and instability. Additionally, frequency conversion generally requires polarized laser sources, which are difficult and costly to assemble using nonstandard polarization maintaining components.